The present invention relates to a heat sealing device, and more particularly to a heat sealing device useful for simultaneously sealing and severing thermoplastic films.
It is well known to those of skill in the art to use a hot-wire sealer or impulse sealing device to simultaneously seal and sever thermoplastic films. FIGS. 1-2 show a typical prior art impulse heat sealing device 8 in the open and closed positions, respectively. Heat sealer 8 includes opposing front jaw 10 and rear jaw 12. First and second films 30 and 32 are inserted between the opposing jaws. Front jaw 10 includes jaw base 16 on which is mounted an insulating layer 18 of polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d) coated woven fiberglass. Heat sealing element 20, which is typically a nichrome resistance wire, is adjacent the insulating layer 18. Front jaw release sheet 22, which is a PTFE-coated woven fiberglass, covers the heating element 20. Because the front jaw release sheet 22 includes a woven fiberglass substrate made up of relatively stiff glass yarns, the release sheet forms voids or air pockets 24 on either side of the heating element 20. However, if the front jaw release sheet 22 were to be conformed around heating element 20 to reduce or eliminate the voids 24, then the glass yarns of the woven fiberglass substrate may break, fracture, or tear during the heat seal operation, thus reducing the effectiveness and life of the front jaw release sheet 22.
Rear jaw 12 includes rear jaw base 26 on which is mounted resilient pad 14, typically made of silicone rubber. Rear jaw release sheet 28 is a PTFE-coated woven fiberglass that covers pad 14. The cross-sectional thickness of resilient pad 14xe2x80x94taken from the rear jaw base 26 to rear jaw release sheet 28xe2x80x94is greater than the cross-sectional diameter or thickness of heat sealing element 20. For example, the pad thickness is more than twice as great as the heat seal element cross-sectional diameter. This difference in thickness allows heating element 20 to press into the pad 14 during the sealing operation, as discussed below.
In operation to form heat seals 36, the first and second jaws move toward each other so that front jaw release sheet 22 engages first film 30 and rear jaw release sheet 28 engages second film 32. The voids 24 and the relative stiffness of front jaw release sheet 22 cause the films in the contact area 34 to conform to a minimal portion of the total surface area of heating element 20. Thus, films 30 and 32 are subjected to pressure only in the relatively narrow localized area 34 rather than broadly conforming to the heating element 20. Typically, the films 30, 32 conform to less than 20% of the total surface area of heating element 20. This small amount of conformity in contact area 34 causes the resulting heat seals 36 to have a relatively narrow width.
Once the front and rear jaws have closed on films 30 and 32 (FIG. 2), the resiliency and thickness of pad 14 causes the pad to apply a relatively constant or uniform pressure to the films 30 and 32 where they conform to the shape of heating element 20. This relatively uniform pressure across the contact area 34 often causes the films 30 and 32 to tear in the area 34 rather than cleanly severing under the heat and pressure of the heating element. The resulting seal/cut has been found to be inadequate for some applications, for example, where a sealed bag contains fluids rather than solids or is required to be impermeable.
The problems discussed above are overcome in the present inventive heat sealing/severing device for heat sealing at least two thermoplastic films together. The device includes front and rear opposing jaws that are moveable between an open position and a closed position. The open position defines a zone for inserting the films between the front and rear jaws. In the closed position, the front and rear jaws are proximate each other to compress the thermoplastic films together. The rear jaw includes a resilient portion facing the front jaw. A heating element is positioned between the insertion zone and the front jaw. In a first aspect, the heating element has a cross-sectional thickness no less than 0.55 times the cross-sectional thickness of the resilient portion.
In a second aspect, a front jaw release sheet is positioned between the insertion zone and the front jaw when the front and rear jaws are in the open position. The heating element is positioned between the front jaw release sheet and the front jaw. In one embodiment of this aspect, the front jaw release sheet engages the heating element when the front and rear jaws are in the closed position and disengages from the heating element when the front and rear jaws are in the open position. In a second embodiment of this aspect, the front jaw release sheet is unreinforced. In a third embodiment of this aspect, the heating element has a cross-sectional thickness no less than about 0.55 times the cross-sectional thickness of the resilient portion.
The present invention includes a method of simultaneously sealing and severing at least two thermoplastic films by the following steps. First, insert the thermoplastic films in the insertion zone of the device of the present invention. Next, move the front and rear jaws to the closed position. As a result, the thermoplastic films are pressed together between the front and rear jaws. Then, apply an electrical impulse to the heating element to increase the temperature of the heating element to a point sufficient to simultaneously sever and heat seal the thermoplastic films. Next, discontinue the electrical impulse to the heating element while the front and rear jaws are in the closed position to set the heat seal.
The heat sealer of the present invention provides several advantages. The front jaw release sheetxe2x80x94which prevents the heating element from sticking to the heat sealed filmsxe2x80x94has a longer effective life because it may engage the heating element only while the heat sealer is in the closed position. Further, the use of an unreinforced release material (e.g., skived PTFE) for the front jaw release sheet allows the release sheet to better conform to the heating elementxe2x80x94thus reducing the size of the voids that are present on either side of the heating element. As a result, the films better conform to the heating element and thus form broader heat seats than a comparative prior art heat sealer. Further, the use of unreinforced release materials for the front and rear jaw release sheets eliminates the chance that reinforcement substrate fibers may tear or otherwise interfere with the sealing and severing process.
The present heat sealer can form consistent heat seals with fewer leaks. The heat sealer/sever device may be used to heat seal two or more films, for example, films as thick as up to 0.010 and 0.015 inches. The heat sealer is especially useful in sealing and severing relatively thin thermoplastic films, such as those having a thickness of less than 0.004 inches (e.g., from about 0.0006 to about 0.002 inches). The resulting heat seals are relatively impermeable, and are suitable for containing fluids. This is in part because the thickness of the heating element may be at least as great as that of the resilient portion of the rear jaw. Once the heat sealer is closed, the resilient portion of the rear jaw applies a compressive force to the films that inherently decreases as the distance from the apex of the heating element increases. As a result, the softened or melted film material is extruded away from the apex area into the heat seal region to reduce voids or leaks and enhance seal consistency. Yet, the heat element cleanly severs the film at the heating element""s apex.
These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the invention and the drawings.